Zinc oxide is widely used in the various industrial fields such as rubber accelerators, pigments for coating compositions and inks, such electronic components as ferrite and varistor, medicinal products and cosmetics. As one of various applications of this zinc oxide, an exoergic filler has been proposed (see Japanese Kokai Publication 2008-19426, Japanese Kokai Publication Heill-246885, Japanese Kokai Publication 2007-70492, Japanese Kokai Publication 2002-201483).
However, alumina and aluminum nitride are usually used widely as the exoergic filler. Therefore, zinc oxide is hardly put into actual utilization compared to these compounds.
However, alumina has a problem that kneading machines become extremely worn in the production process of exoergic sheets and so on, because the Mohs hardness of alumina is high. Further, it is difficult to add aluminum nitride to a resin in high concentration, because of poor filling property. In addition, aluminum nitride is expensive, so exoergic parts made thereof are expensive. Therefore, new exoergic fillers which are made of other materials than such conventional materials are needed.
Zinc oxide has almost intermediate thermal conductivity between alumina and aluminum nitride and is suitable for use as an exoergic filler. However, zinc oxides which are widely used industrially are fine particles having particle diameter of not greater than 1 μm. Such zinc oxide fine particles are hardly used because the heat resistance between particles is high and exoergic property is insufficient.
On the other hand, in the field of electronic components, a thin film layer formed using exoergic filler containing-liquid products such as exoergic greases and resin compositions, for example, coating compositions. There is a problem that the thin film layer cannot be obtained when coarse particles are contained in the exoergic filter. Coarse particles having a particle diameter of 50 μm or more must not be contained in the exoergic filler to employ the filler in the exoergic grease. Zinc oxide of which particle size distribution is kept under control is unknown to the public.
On the other hand, the method of designing particle size distribution to enable the closest packing by combining two or more fillers of various particle size, in order to achieve better thermal conductivity has been suggested (see Japanese Kokai Publication 2002-201483). Therefore, zinc oxide particle showing sharp particle size distribution and having large particle diameter is required. The method of baking with the use of flux is known for increasing the particle diameter of inorganic compounds. However, the zinc oxide particles obtained by the above mentioned method show broad particle size distribution and cause a problem that coarse particles produced in the baking are mixed into the desired products.